In accordance with the worldwide trend of environmental protection and green regulations, adoption of halogen-free standards is the current environmental trend for the global electronics industry. Countries in the world and the electronic related factory established a mass production schedule for halogen-free electronic products. After the Restriction of Hazardous Substances (RoHS) came into force, substances, such as lead, cadmium, mercury, hexa-valent chromium, polybrominated biphenyl, and polybrominated diphenyl ether, are forbidden from being used in manufacturing electronic products or the spare parts thereof. Printed circuit boards (PCB) form the foundation of electronic products and machinery products. Thus, the first and foremost category of objects scrutinized under the halogen-free policy is printed circuit boards. International organizations put forth strict requirements for the halogen content of printed circuit boards. The International Electrotechnical Commission (IEC) rule 61249-2-21 ruled that, in printed circuit boards, the content of the bromide or chloride need to be lower than 900 ppm and the content of total halogen need to be lower than 1500 ppm. The Japan Electronics Packaging Circuits Association (JPCA) ruled that the content of both bromide and chloride need to be lower than 900 ppm. At this stage, the Greenpeace Organization promotes the green policies to ask all related manufacturers to eliminate polyvinylchloride and brominated flame retardants from their electronic products in order to offer lead-free and halogen-free electronic products. Therefore, manufacturers nowadays are interested in halogen-free materials.
There is a trend of new generation electronic products toward thin and lightweight electronic products and high frequency transmission. Thus, the layout of the printed circuit boards trends toward high density. The material selection for the PCB trends toward increasingly strict selection criteria. For bonding the high-frequency electronic device and the PCB, the base material of the PCB must satisfy both the lower dielectric constant (DK) and the dielectric loss (also called dissipation factor, Df) to maintain the transmission speed and the completeness of the transmission signal. At the same time, to maintain the normal operation function of the electronic device at high temperature and high humidity condition, the PCB must satisfy properties of heat resistance, flame retardancy and low water absorption. Epoxy resin has been widely used for copper clad laminate or the sealing material of the electronic spare parts or the electric machinery due to its excellent properties of being adhesive, heat resistant and formable. From the viewpoint of fire prevention, normally, the flame retardant effect of the flame retardant material is obtained by combining the non-flame retardant epoxy resin and the flame retardant. For example, the flame retardancy can be obtained by introducing halogen into the epoxy resin, especially bromine, to improve the reactivity of the epoxy group.
However, recently, the electronic products trend toward lightweight, small and circuit miniaturization. Under such requirements, using the halide, which has high specific gravity, is not a good ideal form the lightweight point of view. Additionally, after operating at high temperature for a long period, the halide might be dissociated and can corrode the fine wiring. Moreover, after combustion, the scrapped electronic parts will produce harmful compounds, such as halide, which is unfriendly to the environment. To replace the halide flame retardant above-mentioned, a means of using the phosphorus compound as the flame retardant has been developed, such as adding phosphate or red phosphorus into the epoxy resin composition. However, the phosphate will dissociate the acid because of the hydrolysis reaction, and transference resistance will be compromised. The Fire Services Act states that, while it is a good flame retardant, red phosphorus is a dangerous substance, because it produces a small amount of phosphine gas at high temperature and moist environment.
The printed circuit boards manufactured from the conventional copper clad laminate use an epoxy resin and a curing agent as the thermosetting resin composition material. The reinforcing material (such as glass fabric) and the thermosetting resin composition are heating and combining to form a prepreg. Then, the prepreg, the upper and lower foils are laminated at high temperature and high pressure. Generally, in the prior art technology, an epoxy resin and a curing agent, which is the phenol novolac resin having the hydroxyl (—OH) group, are the material of the thermosetting resin composition. After reacting the phenol novolac resin and the epoxy resin, the epoxy group will ring open to form another hydroxyl group. The hydroxyl group itself will increase the dielectric constant and the dissipation factor and is easy to combine with moisture increasing the water absorption.
U.S. Pat. No. 7,255,925 disclosed a thermosetting resin composition using the cyanate ester resin, dicyclopentadiene epoxy resin, silica and thermoplastic resin. This thermosetting resin composition has properties of low dielectric constant and low dissipation factor. However, a halogen-containing (such as bromine) flame retardant needs to be used in this manufacturing process, such as tetra-bromo cyclohexane, hexa-bromo cyclodecane and 2,4,6-Tri(tribromo-phenoxy)-1,3,5-triazine. The bromine-containing flame retardant causes the environment pollution during manufacturing, using or even recycling and abandonment. To improve the heat resistance, flame retardancy, low dissipation factor, low water absorption, high cross-linking density, high glass transition temperature, high connectivity and appropriate thermal expansion of the copper clad laminate, the epoxy resin, curing agent and reinforcing material become the main factors for the material selection.
As for the electrical property, the main consideration is the dielectric constant and the dissipation factor of the material. Generally, since the signal transmission speed of the laminate is inversely proportional to the square root of the dielectric constant of the laminate material, the dielectric constant is as small as possible. On the other hand, the smaller dissipation factor indicates the smaller signal transmission loss. Thus, the material having the smaller dissipation factor provides the better signal transmission quality.
Therefore, a material with low dielectric constant and low dissipation factor for manufacturing the high frequency printed circuit board is the problem that the PCB material suppliers want to solve at this stage.